1. Field of the Invention
The present invention relates to telecommunication systems conforming to the cdmaOne standard.
2. Description of the Related Art
The cdmaOne.TM. communication standard is an interim standard for mobile telecommunication systems in which communications to and from each mobile unit are supported by one of a set of base stations strategically located over the system coverage area. The cdmaOne standard specifies a common air interface for code division multiple access (CDMA) systems on the cellular (900 MHz) and the PCS (1900 MHz) bands for mobile telephony. In addition, the same air interface is used for different wireless loop equipment supplied by a significant number of manufacturers. The term "cdmaOne" is used to refer collectively to the IS-95, IS-95A, and IS-95B family of communication standards.
The cdmaOne standard specifies that the symbols encoded in both the forward-link signal transmitted from the base station to the mobile unit and the reverse-link signal transmitted from the mobile unit to the base station be interleaved in the signal stream. Interleaving is performed to make burst errors during transmission look like random errors that are separated from one another in the de-interleaved symbol stream. In that case, the decoder in a receiver can perform error correction to reconstruct the original symbol stream notwithstanding the presence of burst errors.
According to the cdmaOne standard, a base station transmits forward-link data on a Pilot channel (used for timing acquisition), a Sync channel (used for synchronization) at 4800 bps, Paging channels at either 9600 or 4800 bps, and four Traffic (or Fundamental) channels at 9600, 4800, 2400, and 1200 bps. Each frame in a forward-link Sync channel is 26.67 ms in duration and contains 64 different symbols. Each symbol is repeated so that 128 symbols are transmitted per frame at 4800 symbols per second. The data rates of 9600, 4800, 2400, and 1200 bps correspond to the set of four unpunctured rates under the cdmaOne standard referred to as Rate Set 1.
The cdmaOne standard also supports a second set of data rates referred to as Rate Set 2. In Rate Set 2, punctured convolutional codes are used to transmit data at 14400, 7200, 3600, and 1800 bps, corresponding to the unpunctured rates of 9600, 4800, 2400, and 1200 bps, respectively. By using punctured convolutional codes, the number of symbols per frame is maintained, and the interleaving structure for the four rates of Rate Set 2 is the same as the interleaving structure for the four rates of Rate Set 1.
Since only null data is sent on the Pilot channel, no interleaving is used on this channel. However, the cdmaOne specification does require interleaving for the rest of the forward-link channels.
For example, the cdmaOne standard specifies the forward-link interleaving process at the base station for the Sync channel by means of a table. FIG. 1A shows the order in which the 64 different symbols of each frame of un-interleaved forward-link Sync data may be sequentially (or linearly) arranged within a matrix of 16 rows and 8 columns in the base station. The symbols are written columnwise, beginning with the first column on the left, successively from the top row to the bottom row.
FIG. 1B shows the order in which the 64 different symbols stored in the matrix of FIG. 1A are to be read in order to form a frame of interleaved forward-link Sync data. The sequence of symbols in FIG. 1B are listed columnwise, beginning with the first column on the left, successively from the top row to the bottom row. Thus, the symbol #1 in FIG. 1A is the first symbol in an interleaved frame, followed by the symbol #33, followed by the symbol #17, etc.
The de-interleaving process at the mobile unit must perform the reverse of these operations to recover a de-interleaved symbol stream for subsequent processing. Although the cdmaOne standard does not specify the de-interleaving process, typical existing telecommunication systems implement the reverse-link de-interleaving process by an algorithmic deconstruction of the interleaving process. As such, both the interleaving process in a base station and the de-interleaving process in a mobile unit can be implemented at a reasonable cost only in software.